This invention relates to aluminum base alloy products, and more particularly, it relates to improved lithium containing aluminum base alloy products and a method of producing the same.
In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium containing alloy have both improved fracture toughness and strength properties.
However, in the past, aluminum-lithium alloys have exhibited poor transverse ductility. That is, aluminum-lithium alloys have exhibited quite low elongation properties which has been a serious drawback in commercializing these alloys.
These properties appear to result from the anistropic nature of such alloys on working by rolling, for example. This condition is sometimes also referred to as a fibering arrangement, as shown in FIG. 9. The properties across the fibering arrangement are often inferior to properties measured in the direction of rolling, for example. Also, properties measured at 45.degree. with respect to the principal direction of working can also be inferior. By the use of 45.degree. properties herein is meant to include off-axis properties, i.e., properties between the longitudinal and long transverse directions, because the lowest properties are not always located in the 45.degree. direction. Thus, there is a great need to produce a lithium containing aluminum alloy having an isotropic type structure capable of maximizing the properties in all directions.
With respect to conventional alloys, both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys", Properties Related to Fracture Toughness, ASTM STP605, American Society for Testing and Materials, 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys would find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings. Thus, it will be appreciated that obtaining qualities such as high strength at little or no sacrifice in toughness, or where toughness can be controlled as the strength is increased would result in a remarkably unique aluminum-lithium alloy product.
The present invention solves problems which limited the use of these alloys and provides an improved lithium containing aluminum base alloy product which can be processed to provide an isotropic texture or structure and to improve strength characteristics in all directions while retaining high toughness properties or which can be processed to provide a desired strength at a controlled level of toughness.